1. Field of the Invention
The present invention relates to a motor controller and an electric power steering system equipped with the motor controller.
2. Related Art
There has hitherto been known an electric power steering system which provides a steering mechanism of a vehicle with steering assist power by driving an electric motor in accordance with steering torque applied to a handle (a steering wheel) by a driver. Although a brush motor has hitherto been used for an electric motor of the electric power steering system, a brushless motor is recently used, as well, from the viewpoint of an improvement in reliability and durability and a reduction in inertia.
In general, in order to control a torque generated in the motor, the motor controller detects an electric current flowing into the motor, and performs PI control (proportional integration control) in accordance with a difference between an electric current to be supplied to the motor and the detected electric current. In order to detect an electric current of two phases or more, a motor controller for driving a three-phase brushless motor is provided with two or three current sensors.
In connection with the present invention, JP-A-2001-187578 describes determination of a d-axis command voltage and a q-axis command voltage by use of a motor circuit equation. JP-A-2000-184773 describes correction of d-axis command current according to a temperature of the motor.
In a motor controller included in an electric power steering system, a current sensor requires detection of a heavy current of 100 A or greater. The current sensor is large in size and hinders miniaturization of the controller of the electric power steering system. Therefore, in relation to the motor controller included in the electric power steering system, reduction of the current sensor is considered to be a challenge to be met. The cost and power consumption of the motor controller can also be diminished, so long as the current sensor is reduced.
Conceivable methods include a method for diminishing the number of current sensors to one and performing feedback control analogous to related-art feedback control; a method for removing all current sensors and performing open loop control (feed forward control) in accordance with the motor circuit equation.
However, the former method has a drawback of, depending on the rotational position of a rotor of the motor, a single current sensor being unable to detect currents of a plurality of phases required to effect feedback control operation, and there arises a problem of control of the motor being discontinuous. Further, the latter method presents a problem of a failure to correctly drive the motor when a variation has arisen in parameters included in the motor circuit equation.